1. Field of the Invention
The present invention relates to a silicon nitride sintered body which is useful as an automobile part or a structural part for a device such as an OA apparatus, can be prepared at a lower cost than that prepared by a conventional method and has excellent mechanical strength and abrasion resistance particularly at ordinary temperatures, and relates to a method of preparing the same.
2. Description of the Prior Art
Silicon nitride ceramics, which are lightweight as compared with general metals and superior in heat resistance, wear resistance, mechanical strength and toughness as compared to other ceramic materials, are extremely balanced materials applicable to various structural parts. Therefore, the development of the applications of such silicon nitride ceramics has progressed as to such materials being used as an engine part for an automobile or a structural part for an apparatus such as an OA apparatus.
Silicon nitride ceramics are mainly prepared by two methods. The first method is adapted to sinter a raw material of Si.sub.3 N.sub.4 powder with addition of a sintering aid, and the second method is adapted to nitride and sinter a raw material of Si powder with addition of a similar sintering aid. The second method is industrially advantageous as compared with the first method since the material cost is reduced due to employment of the low-priced Si powder in place of Si.sub.3 N.sub.4 powder in view of a small machining allowance in final working resulting from small shrinkage and deformation in sintering. When silicon nitride ceramics are prepared by the second method under conditions substantially identical to those for the first method, however, it is difficult to obtain a homogeneous and dense sintered body since unnitrided Si grains or voids tend to remain in the sintered body. Thus, the second method is inferior in practicality to the first method.
In relation to the second method, therefore, a number of attempts have been made for obtaining powder or a nitrided compact having excellent sinterability with no residue of unnitrided Si grains either by heating a raw material in a high-pressure nitrogen gas atmosphere or nitriding or sintering the raw material over an extremely long time for completely nitriding the same.
For example, Japanese Patent Publication No.61-38149 (1986) (Japanese Patent Laying-Open No. 56-22678) discloses a reactive sintering method of adding powder of at least one sintering assistant selected from a group of compounds of elements belonging to the groups IIa, IIIa, IIIb, IVa and Va of the periodic table to Si powder, shaping the obtained mixture, thereafter nitriding the same in a nitrogen atmosphere of 1200 to 1450.degree. C. for obtaining a nitrided compact of 75 to 80 percent in relative density, and thereafter sintering the nitrided compact in a pressurized nitrogen atmosphere of 1600 to 2200.degree. C. Obtained according to this method is a silicon nitride sintered body consisting of fibrous crystal grains of 0.5 to 2 .mu.m in mean grain size and having bending strength of about 600 to 800 MPa. On the other hand, Japanese Patent Publication No. 7-45345 (1995) (Japanese Patent Laying-Open No. 1-100064) discloses a method of shaping mixed powder prepared by adding subsidiary component powder consisting of at least either Nd.sub.2 O.sub.3 or Sm.sub.2 O.sub.3 and Y.sub.2 O.sub.3 to main component powder of Si powder or a mixture of Si powder and Si.sub.3 N.sub.4 powder, nitriding the mixed powder in a nitrogen atmosphere at 1000 to 1500.degree. C. and thereafter sintering the nitrided powder in pressurized nitrogen gas at 1600 to 2200.degree. C. According to an Example, a silicon nitride sintered body having bending strength of about 550 to 920 MPa has been obtained. In either method, however, no dense sintered body can be obtained unless the material is finally sintered in a high-pressure nitrogen gas atmosphere or subjected to hot isostatic pressing (HIP).
Japanese Patent Publication No. 6-33173 (1994) (Japanese Patent Laying-Open No. 62-3074) discloses. a method of previously crushing an alloy of Si powder and a group IIIa element metal and melting/nitriding the alloy at 1250 to 1450.degree. C. for preparing an Si-group IIIa nitride having a small content of residual Si, further crushing the nitride, thereafter adding and mixing Si.sub.3 N.sub.4 powder and a compound of an element belonging to the group IIa, IIIa or IIIb of the periodic table to and with the nitride and sintering the obtained compact. According to an Example, a silicon nitride sintered body having bending strength of about 700 to 1100 MPa has been obtained. Japanese Patent Laying-Open No. 63-103867 (1988) also discloses a similar method. While either method is adapted to previously improve the affinity between the Si powder and sintering assistant powder for improving sinterability, extremely complicated steps are required for preparing the compact, to result in a considerably high cost. In either method, further, at least 80 percent by weight of Si.sub.3 N.sub.4 powder must be mixed in the step of nitriding the alloy of Si and the metal in order to suppress heat generation, and a cost merit resulting from reactive sintering is small.
Further, Japanese Patent Laying-Open No. 8-245267 (1996) discloses a method of heat-treating commercially available Si powder in a non-nitrogen atmosphere at 300 to 800.degree. C. for previously forming holes in Si grains, adding sintering assistant powder consisting of at least one material selected from a group of compounds of rare earth elements and Al, Mg and Ca by 0.1 to 15 mole percent in terms of the elements and at least one hole forming agent selected from a group of compounds of Sc, Ni, V, Cr, Mn, Fe, Co, Cu, Ti, Zn, Ga and Ge by 0.5 to 15 mole percent to the powder, mixing the materials with each other and thereafter nitriding the mixture in a nitrogen atmosphere at 1300 to 1400.degree. C. for preparing a nitrided compact, and sintering the nitrided compact at 1500 to 1800.degree. C. According to an Example, a silicon nitride sintered body having bending strength of about 800 to 1400 MPa has been obtained. In addition, Japanese Patent Laying-Open No. 8-310868(1996) discloses a similar method of previously chemically treating commercially available Si powder and thereafter heat-treating the same in a non-nitrogen atmosphere at 300 to 800.degree. C. However, either of these methods, which requires specific treatment of the material powder, cannot be carried out at a low cost.